Theoretically yes but by then you wont get 32bit OS's anyway. You could in theory keep a machine running with a 32bit OS until you need that sort of on board Ram on a GPU but i recon it would need to be left in a couple of wills first
Also just for the sake of it the machine wouldnt have any actual Ram if the GPU took up all the address space so wouldnt work anyway. Im not 100% sure but i think that the GPU gets given space first and the system Ram is addressed last.
so if you are running a 2gb card now you are limiting your OS to 1.2 ish of Ram.